1. Field of the Invention
The invention relates to an electron-beam exposure method and, more particularly to, such the electron-beam exposure method as to perform electron-beam exposure by correcting an exposure position when a pattern is directly drawn on a semiconductor wafer using an electron-beam exposure apparatus.
The present application claims priority of Japanese Patent Application No. Hei 11-351080 filed on Dec. 10,1999, which is hereby incorporated by reference.
2. Description of the Related Art
FIG. 6 shows an electron-beam exposure apparatus to which a conventional electron-beam exposure method is applied.
An electron-beam exposure apparatus 100 comprises an electron gun 101 for emitting an electron-beam, a first aperture 102 for restricting an amount of the electron-beam emitted from the electron gun 101, the beam shaping deflector 103 for deflecting the electron-beam from the first aparture 102, a second aperture 104 for further restricting an amount of the electron-beam from the beam shaping deflector 103, a reducing lens 105 for reducing the electron-beam from the second aperture 104, an objective lens 106 for focusing, for image formation, the electron-beam from the reducing lens 105 on a semiconductor wafer, (hereinafter simply to as wafer) 200, an alignment deflector 107 (main deflector) disposed inside the objective lens 106, for deflecting the electron-beam in a predetermined direction, a reflected electron detector 108 for detecting an electron reflected from the surface of the wafer 200, an X/Y stage 109 for loading thereon the wafer 200 to position it in X-axis direction and Y-axis direction, a cabinet 110 for housing all of these components, a alignment deflector control unit 111 for controlling the alignment deflector 107, and a stage control unit 112 for controlling the X/Y stage.
The reflected electron detector 108 uses a diode detector to trap a reflected electron (that is, secondary electron) from the reflective wafer surface when electrons are applied onto an alignment mark formed as a recess in the wafer 200, thus detecting a position of the alignment mark based on a difference between an amount of reflection from the alignment mark and that from its surrounding.
A pattern of the electron-beam from the electron gun 101 is determined through the first and second apertures 102 and 104 having their respective predetermined shapes of openings and the beam shaping deflector 103 and is reduced by the reducing lens 105 to a 1/n scale and then focused by the objective lens 106 onto the wafer 200. In addition, the electron-beam is positioned by the alignment deflector 107 and applied onto the wafer 200 set on the X/Y stage 109. This wafer 200 is positioned to a predetermined place by the X/Y stage 109. The beam shaping deflector 103, the objective lens 106, the alignment deflector 107, or like are adjusted using a reference pattern (not shown) provided on the X/Y stage 109. Drawing pattern data or like are stored in a memory 113 or 114 or a dedicated memory (not shown) or like. The alignment deflector 107 uses the drawing pattern data read out, to thereby control the alignment deflector 107, so that a scanning electron-beam (that is, electron-beam deflected corresponding to the drawing pattern data) may be deflected by the alignment deflector 107, thus drawing a pattern. At this point in time, on the wafer 200, the alignment mark formed thereon has been scanned by the scanning electron-beam prior to pattern drawing, and a resultant reflected electron is detected to thereby find the position of the alignment mark. The alignment mark is thus found and, based on this, the pattern is drawn, to provide drawing on a desired place on the wafer 200, thus avoiding misalignment even in a case where a plurality of drawing patterns is superposed one on top of another.
FIG. 7 shows a plurality of alignment marks provided on the wafer 200. FIG. 8, on the other hand, shows a waveform of a detection signal obtained by the reflected electron detector 108. An alignment mark 201 is cross shaped. The alignment mark 201 is detected when it is scanned by a scanning electron-beam 202 in both X-axis and Y-axis directions in the FIG. 8. When the alignment mark 201 is thus detected, then the wafer 200 is moved by the X/Y stage so that the alignment mark 201 may be positioned at a deflection center of the electron-beam. The alignment mark 200 is scanned actually, for example, by the electron-beam shaped into approximately a 1 xcexcmxc3x971 xcexcm square (may be rectangular) through the first and second apertures 102 and 104 and also by the beam shaping deflector 103. During the scanning, a reflected electron resulting from electron-beam scanning is detected by the reflected electron detector 108. In this case, based on differences in step or material of the alignment mark 201, various reflected electron signals are obtained, such as that having a detected waveform 203 (obtained by X-axis directional scanning) shown in FIG. 8. Such reflected electron signal can be differentiated and then processed by an edge method or a symmetry method, to determine location of the alignment mark.
When a plurality of alignment marks such as shown in FIG. 7 is detected, based on the results of detection a shift in chip layout, a gain (magnification), and a rotation (degree of rotation) are calculated. An error in a chip array is corrected based on these calculation results. This method is referred to as a global alignment method. At a same time, as necessary, a mark at four corners of a chip is also detected, to correct chip shape. Following this alignment, the pattern is projected onto the wafer 200 under exposure.
In a case of electron-beam exposure, however, during exposure a drift in position of the scanning electron-beam 202 may develop due to charge-up (charging of vaporized resist when it is stuck to an inner wall) of a column (lens-barrel), fluctuations in an external magnetic field, wafer charge-up (charging due to electron-beam irradiation), or like.
FIG. 9 shows an example of positional drift of the scanning electron-beam 202. A dotted line indicates a case with no positional drift and a solid line, a case with positional drift. Misalignment in superposing occurs if an amount of positional drift of the scanning electron-beam 202 is not negligible during a time from termination of detection of the alignment mark 201 to termination of exposure.
One proposal to solve this problem is disclosed in Japanese Patent Application Laid-open No. Sho 61-142740. By this electron-beam exposure method, a position detecting mark is set on the chip beforehand and is used to perform first positional detection, which is followed by electron-beam exposure onto the chip to subsequently perform second positional detection, to obtain drift amount in position between first and second positional detection operations, based on which drift amount is calculated an exposure-position correction amount as a time-wise function of exposure processing so that it may be reflected on a next operation of electron-beam exposure.
Also, a method of correcting exposure position by detecting a plurality of standard marks provided on the X/Y stage is disclosed in Japanese Patent No. 2788139 (Japanese Patent Application Laid-open No. Hei 5-84246). This method is described as follows.
FIG. 10 shows an electron-beam exposure method by use of the above-mentioned standard marks.
First, a wafer is set on an X/Y stage in an electron-beam drawing apparatus (step 301). Next, a plurality of standard marks (reference marks) formed on the X/Y stage is detected by electron-beam exposure, to store in a memory thus detected position of the standard marks (step 302). Based on detected results of the standard marks (shift amount, magnification, rotation amount, and other conditions), a correction factor is calculated, based on which is performed alignment (step 303). Next, exposure is started to detect the standard marks on the X/Y stage in a certain lapse of time from the exposure starting (for example, in 2-3 minutes) (step 304), to obtain a difference between these results of detection and those obtained at step 302 (step 305). This difference in detected results provides an electron-beam positional drift amount produced from a time point before exposure up to a time point when the standards marks are detected again. This drift amount is superimposed onto a deflection amount of the scanning electron-beam, based on results of which is corrected the position of electron-beam exposure (step 306). Then, it is checked whether a certain lapse of time has passed (step 307) and, if it is decided not so, the exposure is permitted to continue. If it is decided so, on the other hand, it is then checked whether exposure operations are all completed (step 308) and, if it is decided not so, the standard marks are detected again (step 309). Then, the process returns to step 304 to perform subsequent processing repeatedly for next exposure operations.
According to an another conventional electron-beam exposure method, however, by the method disclosed in Japanese Patent Application Laid-open No. Sho 61-142740, based on a drift amount obtained on a basis of the second position detection, a subsequent positional correction amount is determined as a time-wise function, so that any increase in number of chips on the semiconductor wafer or number of times of superposing increases a drift amount in position, thus deteriorating the alignment accuracy.
Also, by the electron-beam exposure method shown in FIG. 10, a drift amount is calculated by detecting the standard marks provided on the X/Y stage, so that wafer charge-up prevents accurate detection of drift in the electron-beam position, thus making it impossible to obtain a sufficient degree of accuracy in alignment.
In view of the above, it is an object of the invention to provide an electron-beam exposure method which can correct position of an electron-beam based on an amount of position of the electron-beam, while reflecting charge-up of a semiconductor wafer used, thus obtaining a sufficient degree of alignment accuracy in superposing.
According to an aspect of the present invention, there is provided an electron-beam exposure method for exposing a semiconductor wafer with a plurality of alignment marks formed thereon, by performing deflecting scanning by use of an electron-beam according to a desired wiring pattern, including steps of:
detecting positions of the plurality of alignment marks before exposure is started, to correct an error in chip array;
detecting a position of a specific alignment mark selected from the plurality of alignment marks at predetermined timing after exposure is started;
calculating an electron-beam positional drift amount based on a positional difference between the position of the specific alignment mark detected at the predetermined timing and the position of the specific alignment mark before the exposure is started;
superimposing the electron-beam positional drift amount onto a deflection amount of the electron-beam, to correct an exposure position for subsequently performing exposure of the semiconductor wafer; and
until the exposure of the semiconductor wafer is completed, repeatedly performing, a plurality of times, exposure of the semiconductor wafer at predetermined timing based on correction of the exposure position of the semiconductor wafer.
In the foregoing, a preferable mode is one wherein detection of the position of the specific alignment mark after exposure is started is performed on a same alignment mark selected from the plurality of alignment marks.
Also, a preferable mode is one wherein detection of the position of the specific alignment mark after exposure is started is performed on such an alignment mark that is nearest a position of exposure in process selected from the plurality of alignment marks.
Also, a preferable mode is one wherein the predetermined timing is a constant time interval.
Also, a preferable mode is one wherein the predetermined timing is either timing when exposure is terminated for each of chips formed on the wafer or timing when exposure is terminated for each stripe on the wafer.
With the above configuration, first the positions of a plurality of alignment marks are detected before exposure is started, to thereby perform alignment and, at a same time, thus detected positions of the alignment marks provide data for comparison used to obtain an amount of drift in the position of the electron-beam. After exposure is started, the position of a specific alignment mark of the above-mentioned plurality of alignment marks is detected at predetermined timing, so that its difference from the position detected before exposure may give an amount of drift in the electron-beam position. This electron-beam positional drift amount is superimposed onto an amount of deflection of the electron-beam, to correct the exposure position, thereby performing correct exposure. Then, until exposure of the above-mentioned semiconductor wafer is completed, positional correction of the electron-beam is repeated at the predetermined timing by detecting the alignment marks. Thus, the electron-beam position can be corrected reflecting wafer charge-up, thus obtaining a sufficient degree of accuracy in alignment.